In the field of gas content testing, calibration gases may be used to calibrate testing equipment to a known value. The calibration gas is a specific, known mixture of compounds in proper proportions and otherwise known values. For example, in the field of vehicle emissions testing, the calibration gas contains the pollutants typical of those emitted by the vehicles undergoing regular inspection as part of license renewal procedures and other procedures.
At various intervals, usually when the testing equipment is started up, the calibration gas is introduced to the test equipment and the test equipment is calibrated to the known values of the calibration gas. It is important that the pressure of the calibration gas not drop below a specific value, or the proper proportions cannot be maintained. When the calibration is done, the calibration gas is turned off and testing begins.
Each time a calibration is performed, the testing device must ensure that the pressure of the calibration gas is above a preset level. Once turned off, if the gas goes below the preset pressure value, the testing equipment must not allow any flow of gas to the tester to begin until a renewed gas source is installed and reaches the predefined set pressure value.